1. Field of the Invention
The present invention relates to optical modules in which an optical element is flip-chip mounted on a substrate having therein a light transmission hole for passing light emitted from the optical element.
2. Description of Related Art
Wire bonding is conventionally used to mount an optical element such as a VCSEL (vertical cavity surface emitting laser) on a substrate. FIG. 6 is a schematic illustration showing a cross-sectional view of a conventional optical module employing wire bonding. As illustrated in FIG. 6, wire bonding involves electrically connecting, using a wire 64, an optical element 61 mounted on a substrate 62 with a wiring pattern 63 (or another element). Such wire bonding as illustrated in FIG. 6 has the following problem: The wire 64 has a non-negligible length, and is thus accompanied by an undesirable inductance. The presence of such an inductance makes adjustment of the characteristic impedance difficult, thus leading to degraded high-frequency characteristics.
To address this problem, attention has been directed to flip-chip mounting in which an optical element is electrically connoted to a substrate via bumps (such as Au bumps and solder bumps). Flip-chip mounting is characterized in that it reduces the footprint size and provides excellent electrical characteristics such as good impedance matching.
In surface light emitting/receiving elements such as VCSELs, typically, the light emitting/receiving aperture and the electrodes are formed on the same surface. Therefore, when a VCSEL is flip-chip mounted on a substrate, light is emitted from the surface on which the bumps (electrodes) are formed and the light is directed toward the substrate.
A method for preventing light emitted from (or received by) an optical element from being blocked by a substrate is to use a transparent substrate (see, e.g., JP-A 2006-23777). However, a problem is that such transparent substrates (typically made of glass or transparent resin) have poor thermal conductivity. As a result, an optical element mounted on such a transparent substrate cannot be sufficiently cooled, thus possibly causing malfunctions.
To address this problem, optical modules have been proposed in which a ceramic substrate is used because of its relatively good thermal conductivity compared with substrates of glass or transparent resin, and a light transmission hole is formed to allow light to pass through the ceramic substrate (see, e.g., JP-A 2008-134492). However, such optical modules employing a ceramic substrate have the following problems.
FIG. 7 is a schematic illustration showing a cross-sectional enlarged view of a principal part of a conventional optical module using a ceramic substrate. As illustrated in FIG. 7, light emitted from an optical element 72 unavoidably diverges as it passes through a light transmission hole 73 formed in a ceramic substrate 74. As a result, some of the diverging light advances obliquely and is blocked by the inner wall of the light transmission hole 73 in the ceramic substrate 74.
One possible solution to this problem is to make the ceramic substrate 74 as thin as possible. However, the ceramic substrate 74 thinner than a certain thickness cannot be used from a view point of the mechanical strength.
More specifically, in the optical module 71 in FIG. 7, a lens is disposed on the surface of the ceramic substrate 74 opposite the surface on which the optical element 72 is mounted. In this structure, the lens is preferably as near as possible to the optical element 72 in order to enhance the optical coupling therebetween.
In particular, when an optical element array (in which multiple optical sub-elements are arranged in an array) is used as the optical element 72, the counterpart lenses of the optical sub-elements in the array need to be small in size compared with lenses used in non-array type devices. Accordingly, the distance between the optical element 72 and the lens in array type devices needs to be still shorter than that in non-array type devices.
FIG. 8 is a schematic illustration showing a plan view of a conventional VCSEL array. A VCSEL array 81 in FIG. 8 has multiple VCSEL elements 82 arranged in a line. Each VCSEL element 82 is provided with electrodes (a cathode electrode 83, an anode electrode 84, and two dummy pads 85). The pitch between adjacent VCSEL elements 82 is about 250 μm in the FIG. 8 example. Thus, when the VCSEL array 81 in FIG. 8 is used as the optical element 72, the diameter of the counterpart lens of each VCSEL element 82 needs to be as small as less than 250 μm. Hence, the distance between the optical element 72 and the lens array needs to be sufficiently short to optically couple all of the light emitted from each VCSEL element 82 with its counterpart lens.
However, as mentioned before, it is difficult to form the ceramic substrate 74 thinner than a certain thickness because the resulting substrate is mechanically weak. Therefore, the distance between the optical element 72 and the lens array cannot be made sufficiently short because the distance is limited by the thickness of the ceramic substrate 74.
A possible structure to further reduce the distance between the optical element 72 and the lens array is to increase the diameter of a portion of the light transmission hole 73 (on the side of the lens array) and inserting a part of the lens array in the enlarged portion of the light transmission hole 73. However, the light transmission hole 73 of the ceramic substrate 74 is formed by boring with a borer, a drill, or the like, and therefore it is not easy to form a light transmission hole 73 whose inner diameter varies stepwise or continuously. In addition, formation of such a light transmission hole 73 with varying diameters requires a greater number of processing steps, thus adding to the manufacturing complexity and cost.